1. Field of the Invention
The invention relates to a light emitting diode (LED) driving circuit and a power converting circuit. More particularly, the invention relates to an LED driving circuit and a power converting circuit having the function of circuit detecting.
2. Description of Related Art
As the increase of luminescent efficiency of LEDs, LEDs are gradually used to replace conventional lighting apparatuses such as light bulbs or tubes. Basically, the brightness of LEDs is determined by the magnitude of the current flowing through LEDs. Accordingly, a suitable circuit design for LED driving circuits is necessary to effectively control the magnitude of the current flowing through LEDs.
FIG. 1 is a circuit diagram of a conventional LED driving circuit. As shown in FIG. 1, LEDs D1 to D2 are coupled in series to ensure the magnitude of the current i0 flowing through each of the LEDs D1 to D2 is substantially constant. There are a transistor T0 and a resistor R0 coupled between the LED D2 and the ground end G. Herein, the resistor R0 is used to generate the feedback signal VFB. The voltage of the feedback signal VFB is equal to the product of the current i0 flowing through the LEDs and the resistance of the resistor R0. The transistor T0 is used to control the magnitude of the current i0 flowing through each of the LEDs D1 to D2.
Furthermore, the LED driving circuit includes an operational amplifier EA0 of which the inverting input end receives the feedback signal VFB, the non-inverting input end receives a reference voltage signal VREF, and the output end is coupled to the gate of the transistor T0. The operational amplifier EA0 and the transistor T0 form a feedback circuit to stabilize the level of the feedback signal VFB, so as to make the level of the feedback signal VFB be equal to that of the reference voltage signal VREF. Namely, the feedback circuit can stabilize the current i0 flowing through the LEDs D to D2 to a predetermined current value.
Generally, LED driving circuits usually provide an open-circuit protection to deal with the condition of open-circuit due to LEDs. When the condition of open-circuit due to the LEDs D1 to D2 occurs, the current i0 flowing through the LEDs D1 to D2 is decreased to zero, such that the source voltage of the transistor T0 is also decreased to zero. At this time, the open-circuit protection is activated to stop the LED driving circuit. Besides, when the condition of short-circuit occurs in some of the LEDs, the current i0 flowing through the LEDs D1 to D2 is increased because the voltage difference of the LEDs D1 to D2 is lowered. At this time, the feedback circuit formed by the operational amplifier EA0 and the transistor T0 can stabilize the current i0 to the predetermined current value.
However, when the transistor T0 becomes invalid, the conventional LED driving circuit can not detect this condition. Accordingly, the conventional LED driving circuit can not provide a suitable protection. Furthermore, in this condition, the conventional LED driving circuit can not provide the open-circuit protection or the short-circuit protection for the LEDs by controlling the transistor T0.